Is a 9mm kidney stone critical? is it large?

Depends on location: Kidney stones cause kidney damage in two ways, one by increasing the risk of urinary tract infections and two, by blocking the flow of urine and causing pressure back-up and damage. It is important to find out the make up of the stones, such as uric acid stones, calcium stones, etc. Treatment to prevent recurrent stones will depend on the underlying reason for stones, including hydration.

Location matters: 9 mm is relatively big for a kidney stone. If it is lodged in your the tube that the urine travels in to get to your bladder (ureter), it would probably block it and cause potential complications and EXTREME pain. Sometimes stones can pass on their own but one of that size may require a procedure called "lithotripsy". Make sure you are followed by a doctor for managing this.